James Watson's 16 Percent

What does it mean to say that he's one-sixth African?

James Watson, the co-discoverer of DNA who claimed in October that black Africans are less intelligent than whites, is himself 16 percent African. That claim comes from an Icelandic genomics company, which analyzed Watson's genome (publicly available online). But aren't we all of African descent? How can you tell that someone is exactly 16 percent of one origin?

Check his code for single nucleotide polymorphisms. Watson's genome consists of 6 billion nucleotides, the DNA building blocks. The vast majority of this sequence—99.5 percent—is identical from person to person, no matter who you are or where your ancestors came from. But every now and then, a single nucleotide will vary. These differences—called SNPs—are peppered throughout our genome, and result from the subtle evolution that took place as different populations of humans stumbled out of Africa. In most cases, SNPs fall in the vast stretches of DNA that do not code for traits, though some are associated with physical appearance. By scanning a representative sample of the SNPs in a given genome, researchers can begin to piece together the different fractions of a person's ancestry.

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Researchers have identified more than 3 million SNPs, often arranged in groups (known as "haplotypes") of those that happen to be close to one another in the genome sequence. Many of these groups tend to show up in people from a particular place; for example, a certain pattern in a certain location might be most commonly observed in African populations. By scanning hundreds of thousands of SNPs, analysts build a model for the most likely heritage of a person based on which populations have genomes most similar to his. In Watson's case, 16 percent of his genome fits the statistical model of a person of modern African descent. From that, we can conclude about one-sixth of his ancestors came from Africa within the last few hundred generations or so.

Such tests are surrounded in controversy. These ancestral percentages are easily confused with cultural definitions of race and ethnicity. (For example, "African ancestry" does not equal "black.") The accuracy of the tests is also debatable. Some critics charge that the sample populations used to determine which SNPs are associated with which ancestries are too narrow. Sequencing technology continues to improve, but even a small percentage of mistakes can lead to a large number of misread bases when scanning tens of billions of nucleotides. (A genome must be read several times for accuracy.) The company that did the sequencing claims that each base was read an average of 7.4 times, but Kari Stefansson, whose company assessed Watson's heritage, says he found enough errors in the public genome to have doubts about whether the 16 percent figure will hold up. For example, he says there are places where it appears that Watson has two X chromosomes, which would make him a woman.